1. Field of the Invention
The present invention is related generally to injection molding metals and more particularly to compositions of metals suitable for processing in plastics injection molding machines.
2. Background of the Related Art
Conventional reciprocating screw injection molding machines are capable of processing/molding most commercial polymers and filled or reinforced polymers. Although desirable, the machines have not been able to mold parts from metal alloys. Die casting or other variations on the casting process have been the standard methods to manufacture 3-dimensional, near net shape parts from metal alloys. Thixomolding is one method that uses some of the characteristics of plastic injection molding equipment to mold magnesium alloys. The machine used in thixomolding differs substantially in design and size from the conventional plastic injection molding machine.
It is desirable to process and mold metallic alloys (especially lightweight alloys such as aluminum, zinc and magnesium) on convention plastic injection molding equipment. There is a large installed base of injection molding machinery worldwide and the operating cost of this machinery is significantly less than is required for casting and foundry type operations.
Metallic alloys typically have a relatively narrow temperature transition between the solid and liquid phases. Even the semi-solid phase typically has a narrow temperature window.
Metallic alloys cannot be processed on standard injection molding equipment in the solid phase or in the semi-solid phase above some fraction solid because the machine is not strong enough to overcome the resistance of the solid or semi solid (with high solids content). Similarly standard injection molding equipment is not well suited to process any material with very low viscosity (e.g. water like). Materials with too low of a viscosity have little resistance to force (a requirement in the standard injection molding machine design) and exhibit a flow pattern which is not ideal for filling a mold cavity (results in voids, difficulty in packing out, and poor mechanical properties). That leaves only a narrow range of the semi-solid region (e.g. 5-30 solids) that is typically practical for molding metals on injection molding equipment that requires thermoplastic type flow. This narrow range of the semi-solid region also corresponds to an acceptable viscosity range that enables injection molding.
In a conventional injection molding machine plastic pellets enter the conveying screw at or near room temperature. They are typically heated down the length of the barrel to 450-700° F. (˜232-372° C.) depending on the type of plastic and the viscosity desired. The barrel is heated externally to help heat the plastic. The induced shear created by the screw and viscous liquid also accounts for much of the heating of the plastic. Typically barrel temperature is controlled in three zones (front, middle and rear . . . and feed). There is typically only a 100° F. (˜37° C.) difference between the front and rear zone temperature set points. However, the material is heated from nearly room temperature to 500-700° F. (˜260-372° C.) over the length of the barrel. The feed area temperature is set above room temperature but lower than the temperature that is required to induce melting so that in this section pellets remain solid while being conveyed to the hotter zones. The material is continuously heating due to shear and the residence time in the heated barrel. Therefore, there is a continual gradient in the material temperature down the length of the barrel from RT to the injection temperature (a difference of 400-700° F. (˜204-372° C.)). The externally applied barrel heat helps to increase the temperature of the material but is doesn't control the material temperature.
There are other characteristics of the injection molding machine that prohibit precise temperature control in additional to the material temperature gradient down the length of the barrel. Since the screw moves forward and backward there is also potential change in temperature of the material do to its rapid movement up or down the barrel length. New material is constantly being fed and discharged so the heating process is always transient. The molding process is not always running or “on cycle”. Downtime for adjustments or problems also changes the temperature profile of the material because the material is typically not moving during these periods. All these factors contribute to not being able to maintain material temperature over a narrow range.
Temperature of the material in process cannot be precisely controlled because of several factors:                a. material is constantly fed and discharged        b. molding is always a transient process (stop/start)        c. material is heated from near room temperature to the injection temperature (e.g. 700° F./372° C.) so there is a temperature gradient in the material down the length of the barrel        d. barrel set point temperatures range only about 100° F./37° C. from front to back . . . but the material must be heated from 70° F./21° C. to e.g. 700° F./372° C. (therefore the barrel set points can influence but not control the material temp)        e. substantial material heat comes from shear forces which are localized at the walls and not uniformly distributed through the material        f. when the machine stops cycling for whatever reason (and material stops being fed/discharge) the heat balance changes        
All these characteristics make it difficult to maintain a metallic alloy in a processable (narrow) temperature regime. These characteristics are less prohibitive when processing plastics because the processable melt range occurs over a much larger temperature range and the resistance/strength of a cooling plastic is much less than that of metal and can often be more easily overcome by the force of the machine/screw.